1. Field of the Invention
The present invention relates generally to a handoff in a BTS (Base Transceiver System) in a mobile communication system, and in particular, to a method of implementing a handoff in a mobile communication system having a planar structure.
2. Description of the Related Art
In general, an MT (Mobile Terminal) is connected to a BTS via a radio link and the BTS is connected to its higher layers via wired links in a mobile communication system. Existing mobile communication systems are connected to the Internet to deliver information on a larger scale, as illustrated in FIGS. 1A and 1B.
FIG. 1A illustrates the network configuration of a conventional mobile communication system. Referring to FIG. 1A, a PDSN (Packet Data Serving Node) or AGW (Access Gateway) 20 interconnected with an Internet 10 is located in the highest layer. A RAN-CN (Radio Access Network-Core Network) 30 is connected to the PDSN/AGW 20. BTSs (Base Transceiver Systems) 60-1 to 60-m are connected to a BSC 40-1 through a RAN-Access Network 50. Each BTS is connected to a BSC in its higher layer and BSCs are matched with BTSs in a 1-to-N correspondence. Hence, one BTS cannot be connected to two or more BSCs simultaneously.
FIG. 1B illustrates a network configuration under consideration for the future generation mobile communication system. Referring to FIG. 1B, all components are connected over a single network. Also in this mobile communication system, BSCs are matched with BTSs logically in a 1-to-N correspondence. From a physical aspect, the system has a planar structure but a hierarchical structure in effect like the system illustrated in FIG. 1. That is, each BSC is connected to at least two BTSs.
FIG. 2 illustrates the logical hierarchical network structure of the mobile communication having a planar structure. In this network, an inter-BSC handoff may be implemented in the same manner as in the above-described hierarchical network structure. This will be described with reference to FIG. 3.
FIGS. 3A, 3B and 3C depict a handoff in the mobile communication system having a planar structure from an outward perspective but a hierarchical structure from an operational perspective.
Referring to FIG. 3A, an MT 80 is communicating with a source BTS #c 60-3. As the MT 80 roams, a handoff may occur. Referring to FIG. 3B, a handoff occurs when the MT 80 moves to a target BTS #b 60-2. At a conventional hierarchical handoff, a BTS is fixed to a specific higher-layer BSC. Therefore, the target BTS #b 60-2 is connected to a BSC #n 40-n which is in turn connected to the anchor BSC #1 40-1 of the MT 80 via a RAN 70.
The above hierarchical handoff will be described with reference to FIG. 4. FIG. 4 is a diagram illustrating a signal flow for performing an inter-BSC handoff in the conventional mobile communication network having a logically hierarchical structure.
Referring to FIG. 4, the MT 80 transmits a Handoff Request message containing its ID (i.e. its temporary number assigned for communication or its telephone number) to the target BTS #b 60-2 in step 100. In step 110, the target BTS #b 60-2 transmits a Handoff Request message containing the IDs of the MT 80 and the target BTS #b 60-2 to the target BSC #n 40-n. The target BSC #n 40-n transmits a Handoff Request message containing the IDs of the MT 80 and the target BSC #n 40-n to the anchor BSC#1 40-1 in step 120. Then the anchor BSC establishes connections to other devices connected to the MT 80 in step 130 and exchanges traffic with the MT 80 in step 140.
As illustrated in FIG. 3B, two paths are established for the MT 80, from the MT 80 to the anchor BSC #1 40-1 via the source BTS #c 60-3 and from the MT 80 to the target BSC #n 40-n through the target BTS #b 60-2. The target BSC #n 40-n is connected to the anchor BSC #1 40-1. Since the two BSCs operate together for the MT 80, unnecessary load is imposed on them. Moreover, the target BTS #b 60-2 transmits a signal to the anchor BSC #1 40-1 via the target BSC #n 40-n, which adds to the load of the RAN 70. As a result, time required for the handoff is prolonged.
When the handoff is completed as illustrated in FIG. 3C, the target BSC #n 40-n transmits a signal to the MT 80 via the target BTS #b 60-2. Since the target BTS #b 60-2 is connected to the target BSC #n 40-n which transmits traffic via the anchor BSC #n 40-1, the capacity load of the RAN 70 is increased and a data flow is delayed. Consequently, the inter-BSC handoff decreases the capacity of the RAN 70 and increases a transmission delay in the network of a planar structure.